Comet Medals are a type of item that appears in Super Mario Galaxy 2. They are white octagonal coins with a golden lining and a picture of a golden comet imprinted on them. Collecting enough Comet Medals will make Prankster Comets appear, like the Prankster Comets in the first game. They are found in the first mission of every galaxy, usually in hard to reach places or requiring the use of a power-up. Once collected, if the player re-enters the level, the Medal will appear to be a clear blue silhouette of the original. It can still be collected, however. It will temporarily be added to the total, but as soon as the level is completed, the total will revert back to its previous value.

There are 49 Comet Medals total, but not every galaxy will encounter a Prankster Comet, even after every Comet Medal has been collected.

Above a column of coins on the Pond Planet. Cloud Mario is needed, or a triple jump or back-flip and spin at the apex of the jump, Fluffy Bluff has no Prankster Comet of its own until the Green Stars are unlocked.

After sliding down the water slide, in the first room with Snoodles, the Comet Medal is located in a small alcove on the left side of the room, just before the next tunnel, Slimy Spring has no Prankster Comet of its own until the Green Stars are unlocked.

This Comet Medal is located on the expert path. After the Toad says "Experts stay right", go down the right path until seeing the Comet Medal and two Bob-ombs. It should be just before the other path crosses back into the course. It unlocks Purple Coins on the Rainbow Road.

If the player collects an already collected Comet Medal and pauses the game, it counts it to the collection. However, when players return to the hub world, the count gets reverted. It is also worth noting that the number of collected Comet Medals can temporarily be 50 if the player picks one after collecting all 49 Comet Medals.